In general, an antenna is installed wireless communication terminals (mobile terminal, personal communication system (PCS), personal digital assistant (PDA), IMT-2000 terminal, wireless LAN terminal, smart phone, etc.) and receives reception signals from the outside and radiates transmission signals to the outside.
That is, the antenna for receiving signals transmitted from the opposite party or transmitting signals to the opposite party is installed in an appropriate location (inside or outside) of the wireless communication terminal, and the communication with the opposite party is achieved via a wireless communication network.
As the wireless communication terminal is miniaturized and lightweight, the antenna that is one of the largest parts of the wireless communication terminal tends to be smaller, considering the receive sensitivity and harmfulness of electromagnetic wave.
A combination of helical antenna and whip antenna is most widely used as the wireless communication terminals. This antenna is an external antenna that is protruded from an outside of the wireless communication terminal. Since the external antenna requires a lot of parts at a region contacting with the antenna, its assembly process and part management are difficult. The antenna may be easily damaged due to an external impact. Also, since orientation and gain of the antenna is insufficient, high-quality of communication cannot be secured.
To solve the problems of the external antenna, a monopole antenna, a loop antenna, or a planar inverted F antenna (PIFA) is built in the wireless communication terminal, as illustrated in FIG. 1. Accordingly, an outer design of the terminal is elegant and the terminal can be miniaturized. Also, the transmission/reception characteristics can be improved. However, the monopole antenna is different to achieve frequency impedance at low frequency band. The PIFA is an internal antenna used for solving the drawbacks of the monopole antenna. However, the PIFA also has a narrow bandwidth and a current density is condensed at a specific location, resulting in high specific absorption rate (SAR).
To solve the drawbacks of the monopole antenna and the PIFA, there is proposed the loop antenna considering the impedance matching and bandwidth characteristic. However, since the loop antenna using half wavelength is very long, there is a limitation in using the loop antenna as the internal antenna of the wireless communication terminal. Also, in the case of the loop antenna, resonance bandwidth characteristic of high-order mode for multi-band is narrow. Therefore, there is a difficulty in operating the loop antenna as an actual multi-band antenna.
Meanwhile, various antennas have been proposed which can solve the size problem of the internal antenna using a stack structure. However, these antennas are limited to the monopole antenna or the PIFA and have not been applied to the loop antenna till now. Also, only the resonance length of the antenna is compensated using the stack structure.
As described above, the general internal antennas (monopoly antenna, loop antennas, and PIFAs) have limitation in size when implementing them in the built-in type. A bandwidth in a high frequency band is narrow and a current distribution in a high frequency band is differently changed in a low frequency band. Therefore, it is difficult to obtain omni-directional radiation patterns.